Method for identifying materials, impurities and related defects with diffuse dispersion transparent objects

ABSTRACT

Diffusely scattering defects are to be detected in items made from transparent material, e.g. drinks bottles. To this end, the items ( 10 ) are inspected using a light source ( 12 ) and a camera ( 16 ), a contrast pattern ( 14 ) being arranged between the light source ( 12 ) and the item to be inspected. Diffusely scattering defects are detected by a decrease in the contrast of the contrast pattern ( 14 ) reproduced through the item.

BACKGROUND OF THE INVENTION

The invention relates to a method for detecting diffusely scatteringmaterials, impurities, deposits, damage or coatings of the surface orfluctuations in the material thickness in items made of transparentmaterial, the items being transilluminated and examined by a lightsource and a camera.

Structureless, semi-transparent impurities of items made of transparentmaterial such as glass, PET, PC, PVC and similar can often not bedetected using conventional image-processing methods. Among suchimpurities are for example milky adhesive tapes, thin lacquer coats,rust and thin partly mineral deposits on the inside or outside of thewall of the items. Furthermore, the detection of changes on the surfaceof the transparent material, such as scratches covering large areas,abrasion traces, surface parts which are etched or sand-blasted as wellas in general coatings which impair transparency is also problematic.Such semi-transparent defects slightly attenuate the light entering thecamera from the light source in a straight line and diffuse it onlyslightly. Observed over a larger area, such a defect scarcely reducesbrightness, particularly as a dispersion of the light which is broughtabout by the transparent material itself must also be taken intoaccount. In processes which work with bright-field illumination, thedetection of such defects or irregularities is therefore scarcelypossible, as the image recorded by the camera shows neither contrasts,contrasts which could be additionally intensified by image processingmethods, nor a great reduction in the brightness of the image.

Dark-field methods which are based on a change in the polarization ofthe light by defect to be detected (EP-A-0 387 930) are often not usabledue to the polarization effects which occur in transparent containermaterials themselves. Other dark-field methods in which the optical axisof the camera stands at a right angle to the direction of illumination(EP-A-0 429 086) can often be carried out only with difficulty due togeometric boundary conditions, and the scatter caused by thesemi-transparent defects is often not large enough for these processes.

SUMMARY OF THE INVENTION

The object of the invention is to enable structureless, semi-transparentimpurities and defects as listed at the beginning to be detected.

This object is achieved according to the invention in that a contrastpattern is arranged between the light source and the item to beinspected and the contrast of the contrast pattern reproduced throughthe item is determined.

Semi-transparent materials with weak diffuse dispersion greatly changethe optical transmission function. The reproduction of a sharp contouris blurred by this and the contrast weakened. With the method accordingto the invention, this is exploited to inspect transparent containersfor semi-transparent diffusely scattering defects. The contrast patternconsists of transparent and non-transparent regions which are sharplydelimited vis-à-vis each other so that when the contrast pattern isilluminated from the rear, maximum differences in brightness and thus amaximum contrast results. With the method according to the invention,such a contrast pattern is arranged between the light source and thetransparent item to be inspected so that the item to be inspected islocated in the beam path between the contrast pattern and the camera,and the contrast pattern is observed through the transparent item, thedetection process otherwise proceeding as in a normal bright-fielddetection process. The contrast structure of the image recorded by thecamera is analyzed using standard methods of image processing. Withoutsemi-transparent defects, the transmission function is hardly disturbed,and in particular the intensity of the contrasts is retained. If on theother hand, a semi-transparent defect is present, the contrast intensityis reduced in the region of this defect, i.e. the brightnessdistribution becomes more even, as the brightness in the transparentregions of the pattern decreases whilst the brightness in thenon-transparent regions of the pattern is increased. The resultingdeviation from the target contrast can be detected as a defect.

The minimum size of the detectable defect corresponds approximately tothe width of the light and dark regions of the contrast pattern. Thiswidth is in turn matched to the resolution of the optical system andpossible distortions through the refraction of the light in thetransparent material of the items.

Preferably, the camera is focussed on the plane in which the contrastpattern is arranged. The maximum contrast intensity is achieved by thisfocussing.

The greater the space between the contrast pattern and the item to beinspected, the more intense the effect of diffusely scattering defectson the contrast. However, the optical distortions of the contrastpattern caused by fractures in the material of the wall of the itemsalso become greater as a result of an increase in this interval. Asuitable compromise must be found here in each individual case.

Fluctuations in the material thickness of the wall of the itemsinfluence the reproduction of the contrast pattern shown in the focalplane of the camera. Such fluctuations act as an additional lensintroduced into the beam path, whereby on the one hand the contrastpattern is shown distorted, and on the other hand the focussing for thecorresponding image spot is lost. Overall, the contrast structure of thereproduction is thereby influenced such that fluctuations in thematerial thickness can be ascertained in similar fashion tosemi-transparent defects by a decrease in the contrast. In this way,embossed structures in the wall of the items can also be located.

Using the method according to the invention, non-transparent defects canalso be detected as these also lead to a change in the contrast. In thisrespect, however, there are no advantages compared with knownbright-field detection methods.

The method according to the invention can be integrated into alreadyexisting inspection apparatuses, as the light source and the camera arealready present. It can also be combined with other inspectionprocesses. In order that the contrast pattern takes effect only with theinspection process according to the invention, it should be visible onlyunder the optical conditions specified there. It must remain invisiblefor all other inspection processes. This is enabled by a spectralseparation of the two inspection processes. To this end, the contrastpattern can be manufactured from a material in which the non-transparentregions block the beams only for the method according to the invention.The non-transparent regions can e.g. be non-transparent only for aparticular wavelength, the contrast structure then being determined forthis wavelength only. For the other inspection processes which require aroughly uniformly illuminated background surface, a different spectralrange is then used in which the transmission of the contrast pattern isroughly uniform.

Alternatively, the inspection processes can also be separated bydifferent polarization of the individual regions of the contrastpattern. For the method according to the invention, an analyzer is thenused, the direction of polarization of which lies perpendicular to thatof the dark regions. The other inspection processes are then carried outwithout such an analyzer so that the whole surface of the contrastpattern shows a uniform brightness.

Embodiments of the invention are shown in the following using thediagram. There are shown in:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 an arrangement for the detection of semi-transparent defects onthe bottom of a transparent drinks bottle;

FIG. 2 an arrangement for the detection of semi-transparent defects onthe side wall of a transparent drinks bottle; and

FIGS. 3a-3 f examples of contrast patterns.

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment in FIG. 1, drinks bottles 10 are subjected to a bottominspection, to which end the drinks bottles 10 e.g. are held andtransported between laterally gripping pairs of belts. These transportapparatuses are generally known (EP-A-0 124 164) and are therefore notshown. The drinks bottles 10 are illuminated from below through acontrast pattern 14 using a flat light source 12, and a reproduction ofthe bottom of the drinks bottle 10 is produced through the opening ofthe drinks bottle 10 using a CCD camera 16.

The contrast pattern 14 is an arrangement of opaque strips 20, radiatingout from a central point, between which transparent fields 21 arelocated. To this end, the contrast pattern 14 consists of a glass sheetonto which black stripes 20 are glued. The contrast pattern 14 isarranged centrally under the bottom of the drinks bottle 10.

The image evaluation procedure is as follows: the reproduction of thebottle bottom is scanned pointwise, in directions at right angles toeach other, by means of the CCD camera 16. The brightness of each imagespot is determined, and bright-dark and dark-bright transitions areascertained by comparison with the brightness of adjacent image spots.Such a transition occurs e.g. whenever the scan crosses the borderbetween a transparent region 21 and an opaque region 20 of the contrastpattern 14. The number of these transitions and the brightness contrastof the transitions is recorded, to which end the brightness contrast isdivided for example into 250 shades of grey. Semi-transparent, diffuselyscattering impurities lead to a shift to transitions with a smallerbrightness difference. Semi-transparent, diffusely scattering impuritiescan therefore be detected by the fact that the number of transitionswith a large brightness difference, i.e. a large number of shades ofgrey, fails to reach a threshold value. This threshold value isdetermined empirically in advance.

In the embodiment in FIG. 2, the drinks bottle 10 is inspected from theside. The light source 12 covers a correspondingly large surface and thecontrast pattern 14 consists of horizontal parallel strips 20, 21according to FIG. 3b. The contrast pattern 14 is in turn arrangedbetween the light source 12 and the drinks bottle 10. The imageevaluation procedure is analogous to that of FIG. 1.

In FIG. 3, examples of contrast patterns 14 are shown. In general, it isexpedient to select a contrast pattern 14 which is changed as little aspossible by the item to be inspected itself. The bottom of a drinksbottle generally shows fluctuations in thickness which are concentric tothe center. Such fluctuations in thickness act as lenses which wouldvery greatly distort a contrast pattern consisting of concentric circlesaccording to FIG. 3e. On the other hand, the radial contrast patternaccording to FIG. 3f is much less distorted. Similarly, a drinks bottlewith a smooth outside would greatly distort a pattern consisting ofvertical lines, at least in its edge region. On the other hand, apattern consisting of parallel horizontal lines is much less distorted.In the individual case, a suitable contrast pattern must if necessary bedetermined by means of tests.

What is claimed is:
 1. A method for detecting diffusely scattering materials, impurities, deposits, damage or coatings on the surface or fluctuations in the material thickness of containers made from transparent material, the containers being inspected using a light source and an image recording device comprising the steps of: arranging a contrast pattern between the light source and the containers to be inspected; producing a reproduction of the containers by means of the image recording device; scanning the reproduction pointwise to determine image spots; determining the contrast of the contrast pattern by comparing the brightness of each image spot with that of adjacent image spots; and comparing the number of brightness contrasts in which the determined brightness difference lies below a pre-set threshold value.
 2. The method according to claim 1, wherein the image recording device is focussed on the plane of the contrast pattern.
 3. The method according to claim 1 wherein the contrast pattern shows transparent regions and opaque regions which are sharply delimited as compared with each other.
 4. The method according to claim 3, wherein the opaque regions block only light of a particular wavelength, and the light of this wavelength is used for the inspection of the containers.
 5. A method for detecting diffusely scattering materials, impurities, deposits, damage or coatings on the surface or fluctuations in the material thickness of containers made from transparent material, the containers being inspected using a light source and a camera comprising the steps of: arranging a contrast pattern between the light source and the containers to be inspected; producing by means of the camera a reproduction of the containers illuminated through the contrast pattern; scanning the reproduction pointwise to determine image points; determining the brightness of each image point; ascertaining bright-dark and dark-bright transitions between each image point and adjacent image points; recording the number of transitions and their brightness contrast and comparing the number of transitions having a large brightness contrast with a predetermined threshold value.
 6. The method according to claim 5, wherein the camera is focussed on the plane of the contrast pattern.
 7. The method according to claim 5 wherein the contrast pattern shows transparent regions and opaque regions which are sharply delimited as compared with each other.
 8. The method according to claim 7 wherein the opaque regions block only light of a particular wavelength, and the light of this wavelength is used for the inspection of the containers. 